Vehicles powered by vaporized fuel, such as gasoline, have been the subject of numerous patents over many years. Examples include co-owned U.S. Pat. Nos. 6,681,749 and 6,907,866. The disclosures of such patents are incorporated herein by reference.
Vaporizing the fuel prior to entrance to the cylinder can lead to improved performance, particularly with respect to substantially improved fuel economy. Running an engine “lean” (i.e., at an air to fuel ratio of greater than 15:1) can lead to improved fuel economy. Accordingly, vaporizing the fuel prior to entering the combustion chamber of the engine allows the engine to run at much higher air to fuel ratios than a conventional engine, which in turn leads to improved fuel economy.
It has been learned, however, that a potential issue may arise with operating at the larger air-to-fuel ratios, in that an undesired increase NOx emissions may result. This is due in part to the fact that the conventional catalyst on an automobile with a gasoline engine is designed to remove NOx with the engine operating at around a 15:1 air/fuel ratio.
However, applicants have found that the amount of NOx actually produced by the engine decreases as the air/fuel ratio increases, and the increase in emissions is a result of the fact that the catalyst cannot reduce even the smaller amount of NOx produced under these conditions. Thus, applicants have learned that by operating an engine at a sufficiently high air/fuel ratio, the amount of NOx formed would be sufficiently low such that the engine could meet emissions requirements, even with a catalyst that was not operating at its optimal conditions (e.g. air to fuel ratio).
Embodiments of the present invention disclose ways to operate a combustion engine at these higher air-fuel ratio (e.g., about greater than 21:1) such that the level of NOx emitted can satisfy existing regulations. One of the advantages of such operation may be that the high air-fuel ratio can allow for substantial improvements in fuel economy. As catalyst technology employed in vehicles improves, however, the embodiments of the present invention may be used to improve fuel economy with other air-fuel ratios (e.g., between 15:1 and 21:1) and still meet emission standards.
Liquid fuel may be viewed as being comprised of fractions that may vaporize at different temperatures. This vaporization can be achieved by initial heating of liquid fuel at a first temperature (e.g. 70° F.) and subsequently increasing the temperature as the differing fractions of the liquid fuel are vaporized and/or decreased vaporization of the fuel is detected. Referred to as fractionation, applicants have learned that generally sequentially supplying fractions of vapors to the combustion chamber may improve efficiency.
Further, through observation and testing, applicants have discovered that vaporized fuel being conveyed to the engine's combustion chamber may be subject to condensation during such conveyance. This can happen, for example, as a result of ambient air that has a temperature below that of the liquid fuel vaporization temperature being mixed with the fuel vapors to lean out the mixture to achieve the desired air:fuel ratio. This may cause the fuel vapors to in part condense and form liquid droplets. To help achieve improved performance, embodiments of the present invention may help to avoid such condensation by elevating the temperature of the vapor and air mixture to a point above that required for vaporization so that the fuel remains in a vaporized form. In other embodiments, the ambient air that is to be mixed with the vaporized fuel may be preheated.
Further such heating of the air supply, vaporized fuel, and/or air-vaporized fuel mixture may also further enhance the flame speed of the fuel/air mixture. This in turn can extend the “lean limit” (i.e., the highest air: fuel ratio where the engine can perform satisfactorily, without excessive loss of power, misfire, and/or unacceptable hydrocarbon emissions). This extension of the lean limit may have several advantages, including, but not limited to: (1) It improves fuel economy. (2) It decreases the amount of NOx produced.
It is further helpful to elevate the temperature of the ambient air that is mixed with and conveys the fuel vapors from the vaporization chamber prior to entry into the combustion chamber of the engine. As will be discussed more fully hereafter, the mixed air/fuel from the vaporization chamber is further diluted with ambient air, and such further ambient air is also heated whereby the diluted vaporized fuel mixture, upon entering the combustion chamber, is elevated above the temperature of the non-diluted mixture conveyed from the vaporization chamber. As described above, this heating of the air/fuel mixture may help to achieve some of the benefits that improve engine performance, including preventing condensation of the fuel and increasing the flame speed.